The balance between damage and repair of genomic DNA is vital to both the pathogenesis and the treatment of malignancies. When damage to genomic DNA exceeds the ability to repair that damage the consequences can be mutagenic, carcinogenic or even lethal. This is clearly seen in individuals with genetic deficiencies in DNA repair processes such as Bloom's and other syndromes. These individuals have a high predilection for developing chromosomal abnormalities associated with a variety of cancers. Conversely there is also a benefit to be gained when damage to genomic DNA exceeds the ability to repair that damage. Lethal damage is imposed upon genomic DNA by many chemotherapeutic agents. Thus, the enzymes of DNA recombination and repair are especially useful targets for primary and adjunctive chemotherapy. Further development of these enzymes as targets for chemotherapy will depend upon a more complete understanding of the DNA ligand recognized by these enzymes and how various enzymes co-ordinately act to repair damaged DNA. Some of these enzymes probably act upon DNA alone. However, it has become clear that many proteins that bind to, modify or regulate DNA function do so as part of multi-protein complexes. It will be quite important to identify the DNA repair enzymes that associate into complexes for the rational design of ligands that can serve as chemotherapeutic agents. This laboratory has recently identified the formation of a human DNA repair complex that assembles on a unique DNA ligand. A trio of human nuclear proteins binds to the recombinogenic Chi (5'-GCTGGTGG-3') sequence and was affinity purified in a single step. The trio contains a Chi- binding protein and two associated proteins. The Chi binding species was identified as the DNA repair enzyme poly (ADP-ribose) polymerase. The second species forms a stable adenylate adduct and its electrophoretic mobility suggests it is DNA ligase l. The third species co-sediments with the ligase and is probably a newly described "inhibitor" of ligase. Under defined conditions the complex acts like a Chi sequence-specific recombinase. This proposal seeks to further characterize the biochemical and biological activities of this new human DNA repair complex. The complex may be involved in the pathogenesis of oncogenic chromosomal translocations and serve as a target for a new class of chemotherapeutic drugs.